Lithium ion secondary batteries have advantages such as high energy density, small self-discharge, excellent long-term reliability and the like, and therefore they have been put into practical use in notebook-type personal computers and mobile phones. More recently, the development of the high performance lithium ion secondary battery excellent in cycle characteristics and further improved in capacity and energy density is demanded due to, in addition to the trend of high functionality of electronic equipment, the expansion of market of motor driven vehicles such as electric vehicles and hybrid vehicles and the acceleration of the development of domestic and industrial power storage systems.
Attention has been drawn to metallic active materials such as silicon, tin and alloys and metal oxides thereof as a negative electrode active material which provides a high capacity lithium ion secondary battery. These metallic negative electrode active materials provide a high capacity, but the expansion and contraction of the active materials are large when lithium ions are absorbed and released. When charge and discharge are repeated, the negative electrode active material particles collapse due to volume change upon the expansion and contraction, and a flesh active surface thereof is exposed to outside. This active surface decomposes electrolyte solvents and degrades the cycle characteristics of the battery. In order to prevent such disintegration of the metallic negative electrode active material particles, it has been proposed to reduce their particle size.
For example, Patent document 1 discloses a preferred particle size of silicon and/or silicon alloys used as a negative electrode active material of the lithium ion secondary battery. According to Patent document 1, it is preferred that the average particle size of the negative electrode active material particles is 20 μm or less because a negative electrode current collector is directly stressed by the volume change of the negative electrode active material particles in charge and discharge and the negative electrode mixture layer is easily peeled off from the negative electrode current collector. On the other hand, when the particle size of the negative electrode active material particles is too small, the surface area of the negative electrode active material particles per unit weight increases, the contact area with the non-aqueous electrolyte solution increases, irreversible reactions increase, and the capacity decreases. For this reason, it is preferred that the average particle size of the negative electrode active material particles is 1 μm or more.